This invention pertains to low voltage detection circuits and, more particularly, to a combination low voltage detection and alerting circuit that utilizes a relaxation oscillator.
In the design of battery operated radios, it is desirable to provide the user with some indication that battery charge is near depletion. To achieve higher reliability and to keep the cost of the radio at a minimum, it is also desirable to utilize a minimum number of inexpensive discrete electronic components. Accordingly, the invention described below utilizes a single unijunction transistor to output a pulsating current to an LED annunciator circuit when battery voltage drops below a predetermined trigger voltage. This pulsating current causes a light emitting diode (LED) to flash, thereby informing the user that battery charge is near depletion.
A prior art relaxation oscillator is illustrated in FIG. 1. Referring to this figure, the gate (G) voltage of a programmable unijunction transistor (PUT) 102 is established by resistors 104 and 106, which are interconnected as a voltage divider across battery 108. Capacitor 110 is charged through resistor 112. When the voltage across capacitor 110 (i.e., the voltage of the anode of PUT 102) equals the peak voltage of the PUT (the peak voltage being the gate voltage plus a small threshold voltage of approximately 0.35 volts) the PUT switches to a negative resistance mode of operation and capacitor 110 is discharged through the anode to cathode (K) path of the PUT. As the capacitor discharges, current flows through resistor 114 and a voltage pulse is developed at output terminal 116. This cycle is repeated, resulting in periodic voltage pulses at output terminal 116.